Customary immunological methods for diagnosing diseases associated with production of specific antibodies against any pathogen such as viruses, bacteria, allergens, autoantigens or certain pharmaceuticals are based on the ability of these antibodies to form complexes with antigenic structures on the causative agent. In some of these methods, a sample whose content of specific antibodies is to be tested is contacted with the antigenic structures of the pathogen, these antigenic structures being attached to suitable carrier materials. Specific antibodies which are present in the sample are bound and detected as immune complex with the antigenic structures of the pathogen which are immobilized on the carrier material. It is possible to use for this antibodies or other receptors, for example protein A, which are able to form complexes with the specific antibody in the sample. As a rule, the detection reagent carries a label which makes it possible to establish the amount of the bound specific antibody by measurement techniques. Commonly used labels are: radioactive isotopes, enzymes, fluorescent, phosphorescent or luminescent substances, substances with stable, unpaired electrons, erythrocytes, latex particles, metal sols.
Control and standard sera which contain a defined quantity of the antibodies to be detected are required for these immunochemical methods. A positive control serum of this type makes it possible to test the utilizability of the reagents used in the assay. Standardization and thus comparability of assay results which have been obtained on different days or in different laboratories additionally become possible. Quantitative determinations are possible with the aid of standard sera.
A technique which is frequently practized for the preparation of positive control or standard sera comprises taking blood from patients whose disease is caused by defined causative agents, obtaining the serum, and adjusting the control or standard serum to a particular content of specific antibodies directed against the pathogen by mixing sera from different patients.
The disadvantages of this method are that a sufficient number of people whose blood contains these antibodies must be available. Furthermore, there are often medical reasons for not taking blood from such people, for example in the case of children, or there is a risk that the blood of the patient is infectious and no suitable methods for killing this pathogen are known.